The present invention relates to an automatic focusing apparatus for use with a video camera or the like.
Today, many video cameras or the like for family use are provided with an automatic focusing apparatus to automatically establish a focused image of an object. In such an automatic focusing apparatus, a distance between the object and the camera is measured; and based on the obtained distance, the location of the focusing lens at the moment with respect to the position where the focused image of the object is attained is detected. Depending on the result thus detected, the focusing lens is driven by a driving system such as a motor so as to be moved to the position associated with the best focus.
Apparatuses for detecting the distance between the object and the camera have been incidentally described in the JP-A-45-32747, JP-A-46-285000, and JP-A-58-159517, for example. All of these apparatuses to detect the distance operate by use of light. That is, such an apparatus includes a light emitting section having a projection lens and a light emitting element and a light receiving section having a light receiving lens and a light receiving element. A light emitted from the light emitting section is radiated onto an object, which reflects the light to be received by the light receiving section, thereby determining the distance between the apparatus and the object depending on the light reception state associated with the reflected light received by the light receiving section. The focusing lens is moved thereafter depending on the light reception state of the reflected light from the object. Namely, the light reception state on the light receiving element varies according to the distance of the object. Consequently, the focusing lens is moved to set the light reception state of the reflected light from the object on the light receiving element to a particular state and the distance from the apparatus to the object is measured depending on the distance traveled by the light receiving section. As a result, even when the object is located at an arbitrary distance from the apparatus, if the focusing lens is found at a position of the best focus, the light reception state of the light receiving element is kept remained in the particular fixed state.
In the conventional automatic focusing apparatuses, the light receiving element undergoes a parallel displacement or a reflection mirror is rotated to keep the fixed light reception state of the light receiving element so as to detect the distance of movement of the light receiving element or the angle of rotation of the reflection mirror, thereby moving the focusing lens to the position associated with the focused object according to the distance or the angle thus detected. As a consequence, the accuracy of the determination of the position to which the focusing lens is to be moved depends on the distance of movement of the light receiving element and the magnitude of the rotation angle of the reflection mirror. Incidentally, the method in which the reflection mirror is rotated is attended with a disadvantage that the accuracy of the focusing operation is low because of the small angle of rotation of the reflection mirror. On the other hand, in the method in which the light receiving element is moved, the distance of movement of the light receiving element must be increased to improve the focusing accuracy, thereby leading to a disadvantage that the size of the apparatus becomes greater. Furthermore, the light receiving element must be so configured to be movable and to have an initial position adjustable, which leads to a disadvantage that the structure is complicated.
The automatic focusing apparatus according to the present invention is constituted as follows. That is, a light receiving element is mounted in a mechanism in which the initial position is set. After the initial position is established, the light receiving element is fixed on the mechanism. Between the light receiving element and the light receiving lens, there is disposed a flat plate which is rotatable about an axis orthogonal to a plane including an optical axis of the light receiving lens and an optical axis of the projection lens and which allows light irradiated from the light emitting element to pass therethrough. The light radiated from the light emitting element is reflected on the object and the reflected light is refracted when passing through the flat plate. The refracted light enters the light receiving element. If the flat plate is absent, the position where the light enters the light receiving element varies depending on the distance between the apparatus and the object. As a consequence, to attain a constant light reception state in the light receiving element regardless of the distance, the angle of inclination of the flat plate is adjusted. This is because the magnitude of the refraction of the light varies depending on the magnitude of the angle of inclination of the flat plate. As a result, the angle of the flat plate changes according to the distance between the object and the apparatus, which is therefore configured such that the position of the focusing lens is adjusted depending on the angle of inclination of the flat plate, thereby accomplishing the automatic focusing operation.